The exhaust of internal combustion engines is known to contain a number of harmful combustion by-products including carbon monoxide, nitrogen oxides, and unburned hydrocarbons. Manufacturers of cars and trucks continue to make design improvements that reduce such harmful emissions. However, older model vehicles ordinarily do not benefit from subsequent design improvements and pollute at higher levels. Performance of the emission controls can degrade over time, adding to the pollution problems of older vehicles.
One solution to the pollution problems of older vehicles is to retrofit their exhaust systems with filters that remove harmful components of the exhaust. The exhaust filters can be used to either supplement or replace the original emission controls.
U.S. Pat. No. 1,532,730 to Chalupa et al. discloses an early example of an exhaust filter attached to an outlet of a muffler. A conical baffle aligned with the muffler outlet disperses a flow of exhaust through a series of strainers for arresting dust and smoke particles. However, the strainers have little effect on the noxious gasses in the exhaust and could create undesirable back pressure on the engine if clogged by the particulate emissions.
U.S. Pat. No. 3,593,499 to Kile also appends an exhaust filter to a muffler. The filter includes a fibrous disc for removing particles and a replaceable canister filled with charcoal chunks for removing noxious gasses. A fan is positioned in line with the filter to compensate for back pressures produced by the filters. However, the requirement for an independent power source to operate the filter adds undue cost to the design and complicates servicing of exhaust systems.
U.S. Pat. No. 4,706,454 to Smith, Jr. discloses an in-line exhaust filter having primary and secondary passageways. The primary passageway is a conduit interconnecting an inlet and an outlet. The secondary passageway by-passes a portion of the primary passageway for circulating exhaust through one or more annular filter cartridges surrounding the conduit. The exhaust is diverted to and from the secondary passageway by baffle spinners located along the conduit. Although passage of exhaust through the central passageway would reduce back pressures, large portions of the exhaust are expected to remain unfiltered because a pressure drop across the filter would impair return of the exhaust to the conduit.